This invention relates to optical near-field scanning probe microscopy and in particular to a method for performing spectroscopy on a very small, in some cases sub-micron region of a sample surface.
Optical spectroscopy is a useful tool in many analytical fields such as polymer science and biology. Infrared spectroscopy, for example, is a benchmark tool used in science and industry to understand molecular composition and structure in fields including biology, chemistry and material science. Conventional optical spectroscopy and microscopy, however, have been limited by optical diffraction. For visible spectroscopy, the spatial resolution is typically limited to a few hundred nanometers, and for IR spectroscopy, spatial resolution is often limited to the scale of many microns. It would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. Such a capability would provide information about the composition of the sample, such as location of different materials or molecular structures. Conventional infrared spectroscopy is a widely used technique to measure the characteristics of material. In many cases the unique signatures of IR spectra can be used to identify unknown material. Conventional IR spectroscopy is traditionally performed on bulk samples which gives aggregate compositional and chemical structure information. When combined with Infrared Microscopy IR spectra may be gathered with resolution on the scale of many microns resolution. Confocal Raman spectroscopy also obtains spatially resolved chemical information with spatial resolution on the scale of hundreds of nanometers. Raman spectroscopy, however, is not currently as widely used as infrared spectroscopy. As such fewer reference spectra and fewer subject matter experts exist as compared to IR spectroscopy. It is therefore desirable. to have the ability to extend the power of IR spectroscopy to spatial scales that have been previously unavailable.
A variety of techniques have been proposed for performing IR spectroscopy on a sub-micron scale, using a combination of a Scanning Probe Microscope (SPM) and various means for delivering radiation to a sample region probed with the SPM. One such technique is described in U.S. application Ser. No. 11/803,421, owned by the assignees of this application. In this technique, pulses of variable wavelength IR radiation, are directed to the sample surface which is simultaneously probed by an SPM. Rapid sample thermal expansion in response to the pulsed radiation induces oscillation (contact resonance) of the SPM probe. The degree of absorption, and thus probe behavior, depends on the thermal absorption properties of the sample, so by varying the radiation wavelength and observing changes in the response of the SPM probe, highly localized information pertaining to spectral absorption may be obtained. Although this technique is quite promising, it does require an IR tunable pulsed light source, and such devices are. neither common nor inexpensive, as well as a complex optical system to deliver the radiation to a restricted region of the sample surface.
Other techniques based on Nearfield Optical Scanning Microscopy (NSOM) have been proposed. NSOM techniques are conventionally achieved in two ways. In transmissive SNOM, light is guided to the sample using a tapered optical fiber which terminates in a sub-wavelength aperture that acts as a near field source of radiation that can be smaller than the wavelength of radiation being used. Spectroscopic properties of samples can be obtained by recording the light that is reflected, transmitted, absorbed, and/or scattered after emerging from the aperture and interacting with a sample. Another form of SNOM works by focusing a far-field source onto the sharp tip of a scanning probe microscope and then measured the radiation scattered from the tip while the tip interacts with a sample. The optical field at the tip is spatially modulated by the tip apex geometry and the tip-sample distance. Depending on the tip coating and sample material the optical field can be locally enhanced or quenched on a length scale of several nanometers as determined by the tip apex geometries. In combination with spectroscopic techniques this can provide spectroscopic properties (e.g. infrared-vibrational) with nanometer spatial resolution. Both approaches, as with the contact resonance technique, require an external IR light source, typically in the form of an IR emitting laser or laser fed system or some other external illumination (coherent or partially coherent or incoherent) such as IR synchrotron radiation, Free-electron laser, or thermal IR sources (as used for conventional FT-IR spectrometers). The use of an external light source typically requires complex opto-mechanical systems employing lenses and/or mirrors to focus the radiation, and actuators to align it with the probe tip and sample. These requirements make the technique experimentally demanding and limits the accessible spectral range to the tuning range of the sources used.
It is highly desirable if SPM based optical spectroscopy could be accomplished without the need to add the expense and complexity of tunable excitation sources and associated optics, and with simplified techniques for decoupling the probe's mechanical response from spectroscopic properties of the sample. Generation of broadband optical radiation in a region very close to the probe tip and sample, combined with spectroscopic analysis in the far-field can provide a powerful tool to overcome limitations of the current instruments. The current invention discloses such a technique.